This invention relates to multichannel sine synthesizer for primary signal production in measuring signal generators comprising a phase accumulator, a low-pass filter, an interposed synthesis channel and a clock control unit for converting a phase increment applied to the phase accumulator into a sine wave signal.
Synthesizers of this kind, i.e. sine wave generators based on the principle of digital signal synthesis, are known. From the magazine "Elektronik 1976", part Il, pp. 106 to 110, principles of digital synthesizers are known. Such synthesizers have a digital phase accumulator, a synthesis channel and an analog low-pass filter. The synthesis channel can be composed of a series arrangement of a sawtooth-to-sine converter and of a digital-to-analog converter (DAC). An enumerated phase increment is converted by the phase accumulator into a sequence of phase values. The advancement rate is then the clock frequency of a clock control unit. On account of the finite range of values of the phase accumulator, according to value a sequence of sawtooth-shaped phase values is obtained at the output of the phase accumulator. With a given clock frequency, the phase increment therefore determines the output frequency of the sine signal. Consequently, with sine synthesizers variable in frequency the output frequency, which is identical to the fundamental frequency of the sawtooth-shaped sequence of phase values, is controlled proportionally through the phase increment. The sawtooth-to-sine converter converts the individual elements of the sequence of phase values occurring on the input side to corresponding sine function values. The digital-to-analog converter converts these sine function values into linearly assigned voltage or current values. The resulting output signal represents a sample-and-hold function of a sinusoidal signal because the individual supporting values are kept constant within the sampling period. Efficaciously, this is realized by means of a clocked transfer memory of the digital-to-analog converter. For a phase increment .phi. larger than or equal to unity, i.e. the least significant bit (LSB) of the phase accumulator output, sampling and clocking periods are identical. The effective amplitude of the sinusoidal signals then depends according to sin(.pi.f.sub.N /f.sub.T)/(.pi.f.sub.N /f.sub.T) upon the ratio between the effective frequency f.sub.N and the clock frequency f.sub.T. The low-pass filter at the output of the synthesizer serves to attenuate the alias frequency components contained in the output signal of the digital-to-analog converter. The component both highest and lowest in frequency lies at a frequency formed from the difference between the clock frequency f.sub.T and the effective frequency f.sub.N. In order to achieve sufficient attenuation with the use of a reasonable amount of filtering means, as a maximum effective frequency f.sub.Nmax of synthesizers variable in frequency, in general a quarter of the clock frequency f.sub.T is fixed so that the critical alias frequency is at least twice the maximum effective frequency f.sub.Nmax and therefore lies a frequency octave above the maximum frequency. The relative effective amplitude is then, according to the aforementioned formula, about 91% of the effective amplitude with low frequencies. An amplitude compensation of up to 100% can be effected with synthesizers variable in frequency by an increase in height of the transmission band of the low-pass filter. An increase of the effective frequency f.sub.Nmax through, for example, f.sub.T /4 with an additional amplitude compensation is limited because of the approximation to the critical alias frequency f.sub.T -f.sub.Nmax.
From European Patent Specification 0 078 588 (corresponding to U.S. Pat. No. 4,454,486), a device and a method for synthesizing a signal waveform are known. In this variation of a multichannel synthesizer operating by means of a combination of digital signals shifted in time, in the end a high-frequency output signal is produced in which the digital signals shifted in time are supplied to a multiplexer and are then finally converted via a digital-to-analog converter into the desired signal. In such a digital synthesizer, with respect to the dynamic range, more stringent requirements are imposed on the multiplexer and on the digital-to-analog converter as the number of sampled channels is increased. More particularly, the time of conversion of the digital-to-analog converter must be adapted to the sampling period of the multiplexer.